基于蓝宝石纳米图形衬底上氮化镓生长的研究

利用两步生长法在蓝宝石纳米图形衬底（NPSS）上生长得到高质量的氮化镓薄膜。通过XRD和SEM对薄膜质量的表征和研究发现,为得到高质量的氮化镓（GaN）薄膜,在NPSS上生长时得到的最优缓冲层厚度为15nm,而在微米级尺寸的图形衬底（MPSS）上得到的最优缓冲层厚度远大于15nm。同时,在NPSS上生长氮化镓薄膜的过程中观察到一个有趣的现象,即GaN在NPSS上生长的初始阶段,氮化镓晶粒主要在图形之间的平面区域生长,极少量的GaN在衬底图形的侧面上聚集生长。这一有趣的现象明显不同于GaN在MPSS上的生长过程。接着,又在NPSS上生长了GaN基LED结构,并对其光电性能进行了研究。     

High-voltage bipolar mode JFET with normally off characteristics

The first realization of a power vertical JFET operated in the bipolar mode (BJFET) with normally off behavior is reported. The structure combines minority carrier injection from the gate region in the on-state, and lateral pinch-off of the channel, due to the built-in voltage, in the off-state. The realized devices show high blocking voltages, up to 900 V, with zero gate bias, and have extremely low on-resistance. Fast switching speeds with forced gate turn-off times as low as 100 ns for devices of 600-V blocking voltages have been obtained.     

MOCVD方法在蓝宝石衬底上生长无应变InAlN/GaN异质结材料研究

In Al N/Ga N heterostructures were grown on sapphire substrates by low-pressure metal organic chemical vapor deposition.The influences of NH3 flux and growth temperature on the In composition and morphologies of the In Al N were investigated by X-ray diffraction and atomic force microscopy.It’s found that the In composition increases quickly with NH3 flux decrease.But it’s not sensitive to NH3 flux under higher flux.This suggests that lower NH3 flux induces a higher growth rate and an enhanced In incorporation.The In composition also increases with the growth temperatures decreasing,and the defects of the In Al N have close relation with In composition.Unstrained In Al N with In composition of 17% is obtained at NH3 flux of 500 sccm and growth temperature of790 °C.The In Al N/Ga N heterostructure high electron mobility transistor sample showed a high two-dimensional electron gas(2DEG) mobility of 1210 cm2/(V s) with the sheet density of 2.31013cm2 at room temperature.     

在蓝宝石图形衬底上生长低穿透性位错的GaN薄膜

使用MOCVD外延系统,采用3D-2D生长模式在圆锥图形蓝宝石衬底上生长GaN薄膜。研究发现3D-2D生长模式能够有效的减少GaN薄膜的穿透性位错,其中3D GaN层的生长条件是关键：低V/III比,低温和高生长压力。为了进一步减少TD,3D GaN层的厚度应该与图形衬底上的图形高度接近。当3D GaN层生长结束时,3D GaN层把图形衬底的图形围在其中,具有倾斜的侧壁和（0001）向的上表面,而图形上基本没有沉积物。在接下来的2D生长过程里,GaN沿倾斜侧面快速生长,使得侧面上的穿透性位错产生弯曲,从而减少GaN薄膜的穿透性位错。经过对3D条件的优化,GaN薄膜的穿透性位错降低到1×108cm-2,XRD测试得到的（002）,（102）半宽分别达到211弧秒和219弧秒。     

蓝宝石衬底上Ka波段AlN/GaN高电子迁移率晶体管

We report the DC and RF characteristics of AlN/GaN high electron mobility transistors(HEMTs) with the gate length of 100 nm on sapphire substrates. The device exhibits a maximum drain current density of 1.29 A/mm and a peak transconductance of 440 m S/mm. A current gain cutoff frequency and a maximum oscillation frequency of 119 GHz and 155 GHz have been obtained, respectively. Furthermore, the large signal load pull characteristics of the AlN/GaN HEMTs were measured at 29 GHz. An output power density of 429 m W/mm has been demonstrated at a drain bias of 10 V. To the authors’ best knowledge, this is the earliest demonstration of power density at the Ka band for Al N/Ga N HEMTs in the domestic, and also a high frequency of load-pull measurements for Al N/Ga N HEMTs.     

Optical and electrical characterization of GaN layers grown on silicon and sapphire substrates

Characterization of the structural, optical and electrical properties of GaN layers grown by two epitaxial techniques (ECR-MBE and MOCVD) using different substrates (vicinal Si111 and sapphire) has been performed. The quality of the samples grown by MOCVD seems to be influenced by the nitrogen source used for the growth. Unintentionally doped MBE samples with n-type concentrations around 10¹⁸ cm⁻³ and Hall mobility of 15 cm² (V s)⁻¹ were studied. GaN films doped with Mg and grown using AlN buffer layers have also been analyzed to study the influence of the thickness of the buffer layer on the optical properties of the GaN epilayer. In the samples with low Mg doping, a thin AlN buffer layer improved the optical quality of the film. In general, all the MBE samples doped with Mg were highly resistive, probably due to a low activation or high ionization energy of the Mg acceptors. Technological issues related to the formation of ohmic contacts on GaN layers are also presented.     

Infrared reflectance analysis of GaN epitaxial layers grown on sapphire and silicon substrates

Infrared reflectance (IR) of GaN grown on sapphire and silicon substrates has been studied both theoretically and experimentally. The theoretical calculation of the IR spectra is based on the transfer matrix method. The IR spectral characteristics influenced by several factors, such as film thickness, incident angle, free carriers, are systematically examined. Combined with experimental results, surface scattering and interface layer effects are also studied. For GaN epilayers grown on sapphire, carrier concentrations and mobility are determined by fitting to the IR reststrahlen band and compared with the Hall measurement. The interface effect is demonstrated to cause a damping behaviour of the interference fringes away from the reststrahlen band. For GaN grown on Si, the IR spectra predicted the large surface roughness of the epilayers. A variation of IR reststrahlen band is correlated to the microstructures of the films, i.e. their polycrystalline nature of the GaN films grown on Si. A three-component effective medium model is proposed to calculate the IR spectra for polycrystalline GaN, and a qualitative correlation between the IR spectra and structure of the film is established. All results show that IR, as a non-destructive method, is efficient for characterising GaN epilayers in semiconductor processing.     

Microwave performance of AlGaN/GaN metal insulator semiconductorfield effect transistors on sapphire substrates

Metal-insulator-semiconductor field effect transistors (MISFETs) from surface-passivated undoped AlGaN/GaN heterostructures on sapphire were fabricated. Measured static output characteristics includes full channel currents (I_dss) of roughly 750 mA/mm with gate-source pinchoff voltages of -10 V and peak extrinsic transconductancies (g_m) of 100-110 mS/mm. Increased surface roughness resulting from a gate recess process to reduce the pinchoff voltage introduces gate leakage currents in the micro-amps regime. With evidence for reduced dc-to-rf dispersion from pulsed gate transfer characteristics, these devices at 4 GHz with 28.0 V bias generated maximum output power densities of 4.2 W/mm with 14.5 dB of gain and 36% power added efficiency     

Effect of the parameters of sapphire substrates on the crystalline quality of GaN layers

The effect of certain types of sapphire substrate treatment on the properties of gallium nitride layers grown by metal-organic vapor-phase epitaxy is investigated.     

Electrical characteristics of AlGaN/GaN metal-insulator semiconductor heterostructure field-effect transistors on sapphire substrates

The electrical performance of AlGaN/GaN metal-insulator semiconductor, heterostructure field-effect transistors (MISHFETs) were studied and compared to passivated and unpassivated HFETs. Record MISHFET current densities up to 1,010 mA/mm were achieved, and the devices exhibited stable operation at elevated temperatures up to 200°C. Higher maximum-drain current, breakdown voltage, and a lower gate-leakage current were obtained in the MISHFETs compared to unpassivated HFETs. The breakdown voltage of these devices exhibited a negative temperature coefficient of 0.14 VK⁻¹, suggesting that a mechanism other than impact ionization may be responsible. Different structures of MIS diodes also reveal that the high-field region at the gate edge dominates the breakdown mechanism of these devices. Gate-pulse measurements indicate the presence of current collapse in the MISHFETs, despite the expected passivation effect of the insulator. However, a striking feature observed was the mitigation of these effects upon annealing the devices at 385°C for 5 min under N₂ ambient.     

Epitaxial growth of nonpolar GaN films on r-plane sapphire substrates by pulsed laser deposition

Single-crystalline nonpolar GaN epitaxial films have been successfully grown on r-plane sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) with an in-plane epitaxial relationship of GaN[1-100]//Al₂O₃[11-20]. The properties of the ~500 nm-thick nonpolar GaN epitaxial films grown at temperatures ranging from 450 to 880 °C are studied in detail. It is revealed that the surface morphology, the crystalline quality, and the interfacial property of as-grown ~500 nm-thick nonpolar GaN epitaxial films are firstly improved and then decreased with the growth temperature changing from 450 to 880 °C. It shows an optimized result at the growth temperature of 850 °C, and the ~500 nm-thick nonpolar GaN epitaxial films grown at 850 °C show very smooth surface with a root-mean-square surface roughness of 5.5 nm and the best crystalline quality with the full-width at half-maximum values of X-ray rocking curves for GaN(11-20) and GaN(10-11) of 0.8° and 0.9°, respectively. Additionally, there is a 1.7 nm-thick interfacial layer existing between GaN epitaxial films and r-plane sapphire substrates. This work offers an effective approach for achieving single-crystalline nonpolar GaN epitaxial films for the fabrication of nonpolar GaN-based devices.     

High-voltage lateral RESURF MOSFETs on 4H-SiC

High-voltage lateral RESURF MOSFETs have been fabricated on 4H-SiC with both nitrogen and phosphorus as source/drain and RESURF region implants. Blocking voltages as high as 1200 V and specific on-resistances of 4 Ω cm² have been obtained, with the high on-resistance attributed to poor inversion layer mobility. Phosphorus is most appropriate for the source/drain implants due to low sheet resistance and contact resistance with low temperature anneals. However, poor activation of low dose phosphorus implants at 1200°C makes nitrogen the preferred choice for the RESURF region     

A normally off GaN n-MOSFET with Schottky-barrier source and drain on a Si-auto-doped p-GaN/Si

We have fabricated an enhancement-mode n-channel Schottky-barrier-MOSFET (SB-MOSFET) for the first time on a high mobility p-type GaN film grown on silicon substrate. The metal contacts were formed by depositing Al for source/drain contact and Au for gate contact, respectively. Fabricated SB-MOSFET exhibited a threshold voltage of 1.65 V, and a maximum transconductance(g/sub m/) of 1.6 mS/mm at V/sub DS/=5V, which belongs to one of the highest value in GaN MOSFET. The maximum drain current was higher than 3 mA/mm and the off-state drain current was as low as 3 nA/mm.     

High-voltage MIS-gated GaN transistors

Transistors with a high electron mobility based on AlGaN/GaN epitaxial heterostructures are promising component types for creating high-power electronic devices of the next generation. This is due both to a high charge-carrier mobility in the transistor channel and a high electric durability of the material making it possible to achieve high breakdown voltages. For use in power switching devices, normally off GaN transistors operating in the enrichment mode are required. To create normally off GaN transistors, the subgate region on the basis of p-GaN doped with magnesium is more often used. However, optimization of the p-GaN epitaxial-layer thickness and doping level makes it possible to achieve a threshold voltage close to V _th = +2 V for the on-mode of GaN transistors. In this study, it is shown that the use of a subgate MIS (metal–insulator–semiconductor) structure involved in p-GaN transistors results in an increase in the threshold voltage for the on-mode to V _th = +6.8 V, which depends on the subgate-insulator thickness in a wide range. In addition, it is established that the use of the MIS structure results in a decrease in the initial transistor current and the gate current in the on mode, which enables us to decrease the energy losses when controlling powerful GaN transistors.     

Recessed-gate structure approach toward normally off high-Voltage AlGaN/GaN HEMT for power electronics applications

A recessed-gate structure has been studied with a view to realizing normally off operation of high-voltage AlGaN/GaN high-electron mobility transistors (HEMTs) for power electronics applications. The recessed-gate structure is very attractive for realizing normally off high-voltage AlGaN/GaN HEMTs because the gate threshold voltage can be controlled by the etching depth of the recess without significant increase in on-resistance characteristics. With this structure the threshold voltage can be increased with the reduction of two-dimensional electron gas (2DEG) density only under the gate electrode without reduction of 2DEG density in the other channel regions such as the channel between drain and gate. The threshold-voltage increase was experimentally demonstrated. The threshold voltage of fabricated recessed-gate device increased to -0.14 V while the threshold voltage without the recessed-gate structure was about -4 V. The specific on-resistance of the device was maintained as low as 4 m/spl Omega//spl middot/cm/sup 2/ and the breakdown voltage was 435 V. The on-resistance and the breakdown voltage tradeoff characteristics were the same as those of normally on devices. From the viewpoint of device design, the on-resistance for the normally off device was modeled using the relationship between the AlGaN layer thickness under the gate electrode and the 2DEG density. It is found that the MIS gate structure and the recess etching without the offset region between recess edge and gate electrode will further improve the on-resistance. The simulation results show the possibility of the on-resistance below 1 m/spl Omega//spl middot/cm/sup 2/ for normally off AlGaN/GaN HEMTs operating at several hundred volts with threshold voltage up to +1 V.     

InGaN/GaN light-emitting diodes with a reflector at the backside of sapphire substrates

Nitride-based light-emitting diodes (LEDs) with a reflector at the backside of the sapphire substrates have been demonstrated. It was found that an SiO₂/TiO₂ distributed-Bragg reflector (DBR) structure could reflect more downward-emitting photons than an Al-mirror layer. It was also found that the 20-mA output power was 2.76 mW, 2.65 mW, and 2.45 mW for the DBR LED, Al-reflector LED, and conventional LED, respectively. With the same 50-mA current injection, the integrated-electroluminescence (EL) intensity of a DBR LED and an Al-reflector LED was 19% and 15% larger than that observed from a conventional LED.     

DC, RF, and microwave noise performances of AlGaN/GaN HEMTs on sapphire substrates

High-performance AlGaN/GaN high electron-mobility transistors with 0.18-/spl mu/m gate length have been fabricated on a sapphire substrate. The devices exhibited an extrinsic transconductance of 212 mS/mm, a unity current gain cutoff frequency (f/sub T/) of 101 GHz, and a maximum oscillation frequency (f/sub MAX/) of 140 GHz. At V/sub ds/=4 V and I/sub ds/=39.4 mA/mm, the devices exhibited a minimum noise figure (NF/sub min/) of 0.48 dB and an associated gain (Ga) of 11.16 dB at 12 GHz. Also, at a fixed drain bias of 4 V with the drain current swept, the lowest NFmin of 0.48 dB at 12 GHz was obtained at I/sub ds/=40 mA/mm, and a peak G/sub a/ of 11.71 dB at 12 GHz was obtained at I/sub ds/=60 mA/mm. With the drain current held at 40 mA/mm and drain bias swept, the NF/sub min/,, increased almost linearly with the increase of drain bias. Meanwhile, the Ga values decreased linearly with the increase of drain bias. At a fixed bias condition (V/sub ds/=4 V and I/sub ds/=40 mA/mm), the NF/sub min/ values at 12 GHz increased from 0.32 dB at -55/spl deg/C to 2.78 dB at 200/spl deg/C. To our knowledge, these data represent the highest f/sub T/ and f/sub MAX/, and the best microwave noise performance of any GaN-based FETs on sapphire substrates ever reported.     

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ? 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.     

High-quality oxide formed by evaporation of SiO nanopowder: Application to MOSFETs on plastic substrates and GaN epilayers

Although good gate oxide of SiO₂ is usually formed by high-temperature thermal oxidation, lowering the temperature for formation of SiO₂ is mandatory for future Si VLSIs, in particular, for flexible ICs, the demand for which has been increasing every year. Vacuum evaporation of SiO powder is an ideal technique not only to form oxide at low temperature but also to form an abrupt interface with the substrate. The latter feature of evaporation is suitable to form thin gate oxide for Si MOSFETs and gate oxide on compound semiconductors. High-quality SiO₂ on compound semiconductors helps development of MOSFETs made of compound semiconductors, which were longed for to be commercially available. The evaporation is not much used to form SiO₂ for MOSFETs in spite of its many advantages, because quality of SiO₂ formed by evaporation of SiO is too poor to be used as gate oxide. Unlike the commercial SiO powder, the newly developed SiO nanopowder, made by thermal CVD using SiH₄ and O₂, consists of spherical particles with sizes less than 50 nm. It does not contain any Si nanocrystals but small molecular Si networks. Such molecular Si networks are easily thermally or optically decomposed. This makes the deposited oxide more free from Si nanocrystals, which usually degrade the insulating property of the oxide. The SiO₂ thin films formed by evaporation of the SiO nanopowder have demonstrated great potential for application to MOSFETs on plastic substrates and GaN epilayers.     

Threshold voltages of normally off MESFET's

The threshold voltage of "normally off" Si-MESFET's for simple dc-coupled circuits is defined and computed with a two-dimensional device model, taking into account carrier velocity saturation and diffusion. The influence of the main parameters, gate length, conducting-layer thickness, and doping is investigated outside the range of Shockley's equations. A threshold voltage of a MESFET with a 1-µm gate lengthU_{T} = 0.2V±10 percent requires a conducting-layer thickness toleranced =0.15µm±3 percent.